An automotive-vehicle hydrostatic transmission generally comprises an axial-piston pump of the variable-displacement type having a control member which regulates the stroke of the pistons which are displaceable within the respective cylinder barrel and which is generally connected with the crankshaft of the vehicle engine.
The two ports of this hydrostatic pump are connected to the two ports of a hydrostatic (axial-piston) motor whose output shaft is, in turn, connected to a load, usually the driven wheels of the vehicle.
It is known to provide, in connection with such transmissions a hydraulic control circuit which includes a filling pump connected between a reservoir and one of the main hydraulic lines between the variable-displacement pump and the hydrostatic motor, this pump being driven by the engine as well.
Other elements of a control system of this type include a metering throttle, usually in the form of a diaphragm, a reversing or changeover valve which can be automatically controlled to change the transmission ratios by altering the displacement per revolution of the hydrostatic pump.
The throttle diaphragm is connected to a line which forms a bypass connected between the filling pump and the control element of the transmission while the line feeding the latter can also include a so-called feed throttle which need not be of the diaphragm type and is located ahead of a check valve.
Hydrostatic transmissions of this type, especially for automotive vehicles, are usually automatically controlled. This control can be effected by the gas pedal for acceleration and/or by the brake pedal for deceleration and actions by the operator are unnecessary and there is no lag as a result of the intervention of the vehicle operator between the need for varying the transmission ratio and the acceleration or deceleration operation.
In a conventional hydrostatic vehicle transmission, an oil stream at a rate dependent upon the engine speed (RPM), e.g. from a control pump driven by the internal combustion engine, is used for control purposes via a controllable throttle valve.
Ahead of this controllable throttle valve, a speed-responsive signal is obtained which is applied as control pressure for adjusting the displacement of the pump of the transmission.
The signal, in terms of fluid pressure, tapped off ahead of the control throttle is temperature and viscosity dependent so that the controls of such conventional hydrostatic transmissions vary with changes in the viscosity of the fluid and, therefore, the temperature. An ideal, temperature and viscosity insensitive throttle cannot be obtained in practice at low cost.
As a consequence, vehicles controlled in the afore-described manner must have their transmissions heated to the desired operating temperature if accurate response to the driving states is to be obtained. When the transmission is cold, even in idling of the engine, a creep is ascertained in the operation of the hydrostatic transmission. When such creep ensues even at low drive torques, when high torques are applied by the engine, considerable risk prevails that the vehicle will jump ahead inadvertently or uncontrollably.
This risk can be diminished by adjusting the drive speed for the cold vehicle. This, however, results in excessive speeds once the vehicle has warmed up.